The present invention relates to a method and system for correcting an offset in a digital system. In particular, such a correction may be required in a digital communication system in which a frequency offset is present.
Digital communication systems are widely used for transmitting and receiving information. Examples of such digital communication systems include mobile telephone systems, for example the European GSM (Global System for Mobile Communications) system and the American D-AMPS system. Digital communication systems are also being developed for television transmissions.
Within the telecommunications community, there is a desire for a single telecommunication protocol to be introduced enabling the use of a single mobile telephone anywhere in the world. Whilst such a system is not yet in operation, it is apparent that the basis for such a system is likely to be one which uses a spread spectrum signal, in the form of a code division multiple access (CDMA) system.
In a CDMA system, a digital data signal is modulated by a second digital signal, the second signal generally having a significantly higher switching rate. Spread spectrum communication systems were originally developed for military and satellite applications but have since been developed for commercial mobile telephone applications. In the United States of America, the Telecommunications Industry Association has introduced the IS-95 CDMA standard for cellular telecommunications.
At a receiver, a radio receiver converts the received radio signal into an electrical signal. This is then mixed with an oscillator signal to recover the analog spread spectrum signal. An analog to digital converter samples the analog signal to obtain a digital signal, which may be an oversampled signal. This digital signal is despread by correlating the signal with the same pseudo-random code used in the generation of the digital spread spectrum signal to recreate the data signal.
In a CDMA communication system, there is a general requirement to perform automatic frequency correction at a receiver to take into account any systematic or instantaneous frequency variations, in particular transmit and receive oscillator frequency offsets. This is particularly the case in mobile systems, where the motion of the receiver with respect to the transmitter may result in a Doppler shift of the signal.
A variety of techniques have been employed in the prior art to provide automatic frequency correction in CDMA systems. One prior art technique is to perform frequency correction at the input to the receiver. In a CDMA receiver, the input to the receiver has the highest sampling rate and hence this solution demands a very high computational requirement. An alternative prior art solution is to perform frequency error detection and correction after the signal has been de-spread, at the symbol level. Such a solution is however severely limited in the range of frequency correction.
European patent application EP-A-0 762 666 describes a CDMA mobile communication system incorporating means for performing a phase error correction in which a transmitted pilot signal is despread and used to determine phase correction signals which are then fed back to a voltage controlled oscillator. In an alternative embodiment, frequency correction is performed by deriving phase correction signals from phase rotation changes of data with no use of pilot signals for frequency control.